The mast motion is due to thermal flexing caused by changes in the illumination of the telescope by the Sun, both from occultation by the Earth each orbit and from self-shadowing, which changes as a function of Solar aspect angle (SAA). The SAA is the angle between the pointing of the spacecraft and the Earth-Sun axis (where SAA = 0 means that the spacecraft is pointing directly at the Sun, and SAA = 180 is directly anti-Sun) and remains constant during an observation. The roll angle does not contribute to thermal variations since the Sun is always kept on the same side of the spacecraft, normal to the Solar arrays, such that the direction of the Sun around the Z-axis does not change[1]}. Therefore, the changes in \(T_x\) , \(T_y\) , and \(\theta \)  over the course of a single observation are both periodic with the orbital period and reasonably consistent in shape after settling due to the SAA remaining constant (see Fig. REF , black). This means that the mast motion is complex but to an extent predictable, since the orbit and SAA are known for each observation. This predictability gives us the tools to compensate for the potential loss of a laser, and to successfully reconstruct an image with just a single metrology laser.
